There are four things which seem to primarily affect the stitch quality: the tension, the needle, whether you have correctly threaded the machine, and the thread. The present invention is concerned with the tension of the thread. In general, the thicker the fabric, higher the tension must be to lift the lower thread up to the middle of the layers of fabric. Having an incorrect tension leads to a poor quality stitch. The prior art teaches three general techniques for determining tension.
U.S. Pat. No. 7,124,697 issued to Foley teaches a digital thread tension monitoring and control device involving a pair of control discs and are separated by a helical spring, as the discs move closer or further the spring expands or contracts and that determines the tension in the thread. The “helical spring” method has been around for a long time and is popular with undergraduate researchers because the physics make for a simple calculation. As the spring is compressed the device reads the compression and correlates that to an average force. The difficulty is that it is not very accurate, with values ranging +/−2 Newtons in recent studies. For example, Carvalho, et al. Adaptive Control of an Electromagnetically Presser-Foot for Industrial Sewing (available at http://repositorium.sdum.uminho.pt/bitstream/1822/10905/1/ETFA2010_HelderCarvalho.pdf). Efforts to make these systems more accurate are in progress.
U.S. Pat. No. 6,595,150 issued to Yamazaki teaches a thread tension control device utilizing a pneumatic cylinder between a pair of control discs. This system teaches away from the current device and was designed to regulate tension in thread as opposed to displaying the tension.
U.S. Patent Application 2004/0000262 filed by Sakakibara teaches a device that measures the angle of the needle to determine and correct tension between the control discs. Like Yamizaki, no theory is offered on how to measure the tension between the thread discs.